how to open an embedded CID jpg

If you save the attachment to your desktop does it have the format xxxx.jpg? If not rename the file accordingly, it should be .jpg as the suffix not :jpg

The above is correct; it'd usually be named something like cid.jpg and not cid:jpg — with a dot, not a colon.

And are you certain that the sender hasn't made some sort of a mistake?

And cid files—if this is somehow not a jpg file—can be various files not usually opened with common image viewers, too. Dental implants are apparently one use of the cid file extension.